The “sea scorpion” – which lived 467 million years ago – has already been named one of the most powerful ocean predators of its time and grew to a length of nearly 6ft (1.8m).

In keeping with its fearsome power and ability to dominate the seas, with its exoskeleton “helmet”, sleek body and long grasping limbs, scientists have given the beast a mythical name: Pentecopterus decorahensis, after the Greek “penteconter” ship rowed by 50 oarsmen into the Trojan War.

But most species of scorpion as we know them are pretty small and, while they pack a nasty sting, are relatively un-terrifying. So what connects them with this newly found prehistoric horror, which sounds more like a killer giant crab?

The exoskeleton, slender body and grasping limbs make the sea scorpion sound more like this guy (Sean Dempsey/PA)

Scientists explain that sea scorpions – or eurypterids – look like crustaceans but are actually the ancestors of modern spiders.

Dr James Lamsdell, lead researcher from Yale University in the US, said: “The new species is incredibly bizarre.

“The shape of the paddle – the leg which it would use to swim – is unique, as is the shape of the head. It’s also big – over a meter-and-a-half long!”

Scorpions as we know them now are more pocket-sized (Ben Curtis/PA)

But as well as the species itself, what has really mystified scientists is its impressive preservation that has kept it hidden in extraordinary detail for so many millions of years.

“The exoskeleton is compressed on the rock but can be peeled off and studied under a microscope,” Dr Lansdell said. “This shows an amazing amount of detail, such as the patterns of small hairs on the legs.

“At times it seems like you are studying the shed skin of a modern animal – an incredibly exciting opportunity for any palaeontologist.”

It took more than 150 fossil fragments to come to a final identification of the creature, all excavated from Winneshiek Shale sedimentary rocks in north-east Iowa in the US.

Studying them has led scientists to believe that it was the largest known eurypterid from its era – with some of its bodily segments as long as 5ft 7in (1.7m) – and 10 million years older than any other discovered to date.

Exceptionally preserved remains of the exoskeleton has also allowed scientists to identify and interpret the role of fine structures such as scales, follicles and stiff bristles. For example, the creature’s rearmost limbs are covered in dense setae, or bristles, which may have contributed to its swimming ability, or had a sensory function.

Spines on some limbs appear similar to those of horseshoe crabs, which use them to aid food processing.

Some of the sea scorpion’s features are similar to creatures such as the horseshoe crab (Peter Galvin/Flickr)

“Pentecopterus is large and predatory, and eurypterids must have been important predators in these early Palaeozoic ecosystems,” said Dr Lamsdell.

During the Ordovician period, when Pentecopterus was alive, invertebrates ruled the oceans and the first animals were only just starting to colonise the land.

Early fish at this time grew no longer than about 12in (30cm) and were jawless. They would have been no match for a giant sea scorpion.